1. Field of the Invention
The present invention relates to an optical interconnect system, and in particular to a backplane interconnect. Such an optical backplane might be used, for example, as a broadband interconnect for a multiprocessor computer system or in a switch for a broadband optical telecommunications system.
2. Related Art
A backplane interconnect system is typically required to provide many parallel data channels running across its width with access points at different locations along its length. Cards carrying interface and signal processing circuits may be fitted to the backplane at the different access points. When such an interconnect is used in a complex system, such as a switch in a telecommunications system, then it may have to accommodate a large number of cards. In these circumstances, if a conventional electrical backplane is used, then the limited bandwidth of the interconnect and crosstalk between different channels on the interconnect become significant factors in limiting the performance of the system. It has been recognised therefore that it would be desirable to provide an optical backplane to overcome these limitations.
It has previously been proposed to construct an optical backplane using laser arrays collimated by microlenses. This offers the required flexibility for the access points, but the channel packing density across the width of the card is limited by diffraction. This makes for inefficient use of the available area and causes the number of channels in a given cross section of the interconnect system to fall with increasing interconnect length. Diffraction can be overcome by using a single macrolens in place of an array of microlenses to collimate the laser array, but then the beams are no longer parallel to the axis and a large receiving lens is required to collect all the light. With a typical laser array [for example 8.times.8 devices, 125 .mu.m pitch, 8.degree. full beam width] and macrolenses with optimised focal lengths, connection densities are still lower than those achievable using microlenses.
An alternative approach is to use image relay systems to overcome the fall in connection density with distance. Certain optical systems may be concatenated to produce repeated images of an array while confining the beams within a constant envelope. Examples of such systems are disclosed in U.S. Pat. Nos. 5202567 and 5362961. The functioning of conventional image relay systems depends critically upon the accurate alignment of a large number of lenses. In the systems disclosed in the above-cited patents, this problem is addressed by forming the entire lens system integrally with an underlying glass substrate. This however makes the process of manufacture relatively complex, and tends moreover to limit the size to which the backplane can extend. U.S. Pat. No. 5,245,630, assigned to Unisys Corporation, discloses an optical interconnect which uses a series of coaxially aligned GRIN (graded refractive index) lenses interspersed with transmitter-receiver repeaters. The optical properties of the GRIN lenses are such that the relay can only function satisfactorily with on-axis images. Off-axis images suffer increasing divergence as the image passes down the relay. As a result, a separate series of lenses is required for each optical source, so that each source may be located on-axis. This has been found to limit unacceptably the connection density which can be achieved.